The present invention is directed to a method for examining a body region executing a periodic motion in an examination subject be means of computed tomography (CT) apparatus of the type having an X-ray source continuously rotating around a system axis and from which an X-ray beam penetrating the examination subject proceeds, and having a detector system for the X-rays emanating from the X-ray source that has at least a first detector line and a last detector line, wherein the examination subject, and the X-ray source and the detector system, are displaced relative to one another in the direction of the system axis during the rotation of the X-ray source, and wherein a tomogram of at least the body region executing the periodic motion is determined with an electronic calculating means from the output data of the detector system corresponding to the detected X-rays.
The technique of prospectively ECG-triggered exposure of individual slices with single-line CT apparatus of the 3rd generation (X-ray source and detector system rotate in common around a system axis) has been known since the early 1980""s. A characteristic feature of the ECG signal, for example the R-wave, is used to implement an axial exposure in a defined heart phase for a fixed position (z-position) of the examination subject relative to the X-ray source and detector system in the direction of the system axis (z-direction). A full-revolution scan or sub-revolution scan is triggered after a selectable time delay (in % of the average RR interval of the ECG signal or absolutely in msec) relative to the respectively preceding R-wave. The data required for the image reconstruction at this z-position can be collected in a number of successive revolutions in order to improve the time resolution.
Given a single-line CT apparatus, i.e. a CT apparatus whose detector system is a single line of individual detectors, it is also known to implement a spiral scan with the ECG signal registered in parallel therewith (referred to as retrospective gating). Here, tomograms are subsequently calculated in the image reconstruction only from data in permitted data regions (in the respectively desired heart phase, for example in the diastole). In a single-line CT apparatus, this has the drawback that a gap-free coverage of the volume z-direction with tomograms is not achieved. In a spiral scan, a relative displacement of the X-ray source and detector system and the examination subject in the z-direction ensues simultaneously with a continuous rotation of X-ray-source and detector system around the system axis.
With the introduction of new CT apparatuses of the 3rd generation with sub-second rotation, i.e. X-ray source and detector system need less than one second for a complete revolution around the system axis, and multi-slice technology, i.e. a detector system with more than one line of individual detectors, heart diagnostics with CT apparatuses is experiencing a new boom. ECG-triggered axial exposures as well as spiral exposures with ECG signal registered in parallel (retrospective ECG gating) have been expanded to multi-slice CT apparatuses, i.e. CT apparatuses with multi-slice technology, which are also referred to as multi-line CT apparatuses. Due to the multi-cell quality, new possibilities also exist in retrospectively gated spiral examination with suitable reconstruction techniques such as, for example, the gap-free presentation of the heart volume in the z-direction in any desired phase of the heart cycle.
In ECG-triggered axial exposures, radiation is only triggered within the time span during which the data actually required for the image reconstruction are registered. The method is thus dose-sensitive; it uses only the X-ray dose actually needed for the image calculation. However, one tomogram of a slice (given single-line CT apparatus) or a number of tomograms of slices (given multi-line CT apparatus) are respectively registered at fixed table positions per exposure (scan). Between two scans, the examination subject and the X-ray source and the detector systemxe2x80x94which assume a fixed z-position relative to one another during a scanxe2x80x94must be brought into the new, desired z-position. This takes time and is the reason that tomograms usually cannot be registered in every heart cycle (heart period) but only in every second or every third. The examination time is considerably lengthened as a result, and it is often not possible to acquire tomograms of the desired, thin slices of the entire heart volume in one breath-holding phase. Given multi-slice CT apparatus, moreover, the tomograms arise automatically at the spacing of a detector line in z-direction. For qualitatively high-grade 3D applications, for example volume renderings for presentation of the coronary arteries, however, tomograms with a smaller spacing in the z-direction are required. A reconstruction of corresponding tomograms is not possible given conventional single-slice exposures.
Given spiral scans with a multi-line CT apparatus having the ECG signal registered in parallel, data are registered during the entire duration of the spiral scan. The data registered in the desired heart phases of the heart cycle are identified later (retrospectively) from the ECG signal registered during the spiral scan and are utilized for the reconstruction. In a multi-slice CT apparatus, this method has the advantage that tomograms in every desired heart phase can be reconstructed overlapping in the z-direction at arbitrarily small spacings. Due to the continuous relative motion between the examination subject and the X-ray source and detector system, the coverage of the entire heart volume with thin slices is possible in one breath-holding phase in a multi-slice CT apparatus. Both features are preconditions for qualitatively high-grade 3D representations of the heart.
In such exposure techniques, the patient absorbs an X-ray dose that is not inconsiderable.
In the context of reducing the dose applied to a patient, U.S. Pat. No. 5,625,662 teaches modulating the tube current of an X-ray tube provided as the X-ray source in a CT apparatus, dependent on the rotational angle of the X-ray tube as well as on weighting factors to be applied to the data acquired at the respective rotational angle.
In the same context, U.S. Pat. No. 5,485,494 discloses a CT apparatus wherein the tube current of an X-ray tube provided as the X-radiation source is modulated dependent on the rotational angle, this modulation being undertaken according to a stored function that is preferably acquired on the basis of a test scan of the patient.
An object of the present invention is to provide a method of the type initially described, wherein a body region of an examination subject executing a periodic motion can be registered with a reduced radiation dose.
The above object is achieved in accordance with the principles of the present invention in a method for examining a body region of an examination subject executing a periodic motion with a CT apparatus having a multi-line detector system in spiral mode, wherein the X-ray source is activated and deactivated for the emission of X-rays substantially synchronously with the periodic motion, so that the X-ray source is activated only during a phase of the periodic motion to be imaged with the CT apparatus.
In the invention, thus, the advantages of a prospective triggering (only the dose actually required is applied) are united with the advantages of the spiral scan with a multi-line CT apparatus (gap-free volume coverage, possibility of overlapping reconstruction of tomograms) for examinations of, for example, the heart in a previously defined (selected) heart phase. To that end, full revolution or sub-revolution scans are prospectively triggered during the individual heart cycles with a selectable time delay relative to the respectively preceding R-wave of the ECG signal (in % or as a fraction of the average duration of the R R interval of the ECG signal or absolutely in msec), with X-rays being emitted only during the time for the registration of the data during the full revolution or sub-revolution scan, with the detector system comprising a number of detector lines (multi-line full revolution datasets or sub-revolution datasets). Thus, not only the data registration but also the X-rays are prospectively triggered, with the X-ray source being correspondingly activated and deactivated. The relative displacement in the z-direction between the examination subject and the X-ray source and detector system is not arrested in the xe2x80x9cstop and goxe2x80x9d operation for the respective scan and undertaken only from one z-position to the next between two scans as in spiral scans. Instead, the relative displacement ensues continuously during the scans as well as in the time between them. Multi-line sub-revolution data or full revolution data are thus obtained wherein each projection corresponds to a different z-position. By applying suitable reconstruction and weighting methods (for example, projection-dependent weighting between the data of the individual detector lines), gap-free tomograms can be reconstructed therefrom in the z-direction within a region that is dependent on the feed velocity, and thus on the pitch. The feed velocity is selected dependent on the period duration of the heart cycles, i.e. on the heart frequency, taking the detector width into consideration such that the regions covered by successive datasets overlap in the z-direction orxe2x80x94in the limit casexe2x80x94abut one another gap-free. The entire heart volume thus can be scanned in one breath-holding phase due to the continuous relative displacement in the z-direction between the examination subject and the X-ray source and detector system, and due to the elimination of the acceleration and deceleration phases required given discontinuous displacement.
The method of adapting the feed velocity to the heart frequency is relevant not only to the inventive method but also to conventional ECG-gated multi-line CT spiral scans.
In a version of the invention, the time duration during which the X-ray source is activated during a heart cycle is longer than the duration of a time interval during which measured data are acquired, i.e. it is longer than the duration of a reconstruction interval or data interval. In the case of fluctuations of the duration of the heart cycle (arrhythmia) it is thus assured that the phase of the heart cycle to be image is in fact registered.
In another version of the invention, the time curve of the ECG signal and the measured data are stored, and the measured data utilized for determining a tomogram are selected, taking the signal into consideration, so that they were acquired during the phase to be imaged. A retrospective ECG-gating thus can also be implemented on the basis of the inventive method.
It is thus clear that, when there is interest only for tomograms of the heart in a specific phase, usually the quiescent phase (diastole), the inventive method is clearly more dose-sensitive than a spiral scan with retrospective gating implemented with a multi-line CT apparatus, since the patient is irradiated with X-rays during the entire spiral scan in the latter case but only a small part of the registered data is in fact used for the image reconstruction.